Electric motors, especially brushless DC motors are used as actuators in medical devices. For example, an electric motor may be provided in a liquid drug delivery device to actuate the drug delivery. The dose of the drug delivered to a patient may then be directly dependent on the motor rotation, so that a precise control of the motor rotation is crucial for these applications. The input signal for the motor control is usually the position of the armature, which in the art is often evaluated optically or magnetically with an additional sensor on the motor. Additional sensors, however, require additional components and therefore increase costs. They also may compromise the device robustness to malfunctions since extra components may cause additional functional failures.
There are known sensor-less motor control systems, which make use of electromotive force measurements. The measurement of the electromotive force works well for high speed (high rpm), but less for low speed (low rpm) motors. For some applications, such as drug delivery devices, it is however necessary to precisely control the motor also at low speed.